Not to be confused with a Cluster F-bomb1 a cluster bomb is a round of ammunition - originally an aerospace bomb2 - which deploys multiple submunitions over the target area, allowing it to attack a much larger radius than it would if it were a purely explosive munition.
The default cluster munition is composed of a number of explosive bomblets which are fused either to airburst or to explode on impact and is intended to clear enemy infantry out of soft cover or to destroy soft-skinned vehicles. More advanced versions airburst some bomblets and have others fused to act as landmines on landing for area denial purposes.
Incendiary versions of this weapon have been in use since the 1930s - the Molotov Breadbasket played a minor role in the Soviet-Finnish Winter war and all sides used similar weapons in the mian conflict of WW2.
Another key type of cluster munition is designed for anti-AFV work: the submunitions are equipped with advanced sensors and some form of aerodynamic retardation so that they drift above the battlefield looking for targets that could plausibly be AFV. On finding themselves above one they launch an explosively formed penetrator attack against the target's (weaker) top armour. As with the anti-infantry version, some designs can convert themselves into mines if they land without being detonated. Theoretically, the search-and-attack function could be added to anti-personnel versions but almost certainly wouldn't be worth the money.
A third key role for cluster munitions is in the destruction and denial of runways - in this case the bomblet load is mixed between concrete penetrating devices designed to break up the runway surface and anti-personnel mines designed to hamper repairs.
Other designs of cluster bomb have been prepared to attack electrical grid systems (mainly by shorting them out with conductant fibres) and to disperse chemical or biological agents. Some authorities also class air-deployed mines as cluster weapons.